With the recent progress of portable or compact equipment, the batteries are also required to reduce their size and weight. Among others, lithium secondary batteries are favorably expected for a high capacity and high energy density. To increase their capacity, improvements in the positive electrode, negative electrode, electrolysis solution and other components have been under study.
Lithium secondary batteries generally use lithium-containing transition metal oxides as typified by LiCoO2, LiNiO2 and LiMn2O4 as the positive electrode active material; and carbonaceous materials such as graphite and coke as the negative electrode active material. Of these, graphite materials are preferably used because their use as the electrode leads to an increase of energy density per volume.
As the electrolysis solution, solvent mixtures of cyclic carbonates as typified by ethylene carbonate and chain-like carbonates as typified by diethyl carbonate or methyl ethyl carbonate are frequently used.
In particular, cyclic carbonates have a high degree of dissociation of lithium salts, and inter alia, ethylene carbonate has a wide potential window and resistant to oxidation and reduction. It is known that using the cyclic carbonate as a main solvent of a non-aqueous electrolysis solution, lithium secondary batteries having improved charge/discharge characteristics can be fabricated.
However, ethylene carbonate has a problem due to a high melting point (about 37° C.) that it solidifies into a solid at low temperatures including room temperature and has a low conductivity at low temperatures. If a large amount of chain-like carbonate is mixed in order to lower the freezing point at low temperatures, the potential expansion and safety hazard of the battery during high-temperature storage due to the low boiling point and flash point of chain-like carbonate become a concern. Also ethylene carbonate undergoes gradual decomposition with increasing cycles, leading to a degradation of cycle performance.
An attempt was made to use propylene carbonate as the cyclic carbonate having a lower melting point instead of ethylene carbonate. Since propylene carbonate has resistance to oxidative and reductive decomposition and a low freezing point (about −49° C.), it is advantageously used when lithium metal or low crystalline carbon is used as the negative electrode.
However, when graphite is used as the negative electrode, the use of propylene carbonate as a main solvent of an electrolysis solution gives rise to the problem that severe decomposition of propylene carbonate on the negative electrode prohibits charging.
Particularly when synthetic graphite having a high degree of graphitization is used to meet a demand for a higher capacity, the phenomenon that propylene carbonate attacks the laminar structure of graphite becomes more outstanding. This prohibits the use of propylene carbonate as a main solvent in a high proportion.
Techniques of adding an additive to control the decomposition of propylene carbonate and graphite have been reported. JP-A 11-73990 proposes to use a solvent mixture of ethylene sulfite and propylene carbonate. This relies on the mechanism that decomposed products of ethylene sulfite form a coating on the negative electrode to control the decomposition of propylene carbonate.
JP-A 2000-3724 describes that 1,3-propanesultone or 1,4-butanesultone is effective for suppressing decomposition of propylene carbonate. However, these techniques are difficult to improve battery characteristics.
Meanwhile, in order to increase the energy density of a battery, the proportion of active material in the entire battery must be increased. One means for increasing the proportion of active material is to increase the amount of active material loaded per electrode area. The increased amount of active material loaded, however, allows for decomposition of propylene carbonate and graphite even when an additive is used. This is probably because the increased amount of active material loaded leads to a thicker electrode which experiences more polarization and makes it difficult to form a uniform coating.
On the other hand, with respect to lithium ion secondary batteries and lithium ion polymer batteries, the trend of development in the art requires to further increase the battery energy density. A strong demand is imposed on an improved volume energy density that a high capacity is packed within a certain space. The battery energy density can be increased by increasing the capacity of positive and negative electrode active materials while the same purpose can be achieved by increasing the density of electrodes even when conventional positive and negative electrode active materials are used.
The energy density of an electrode can be increased by forming an electrode as by coating, then processing the electrode constituting material under a high pressure to provide a low porosity.
However, a problem arises when the low porosity electrode is used in a battery. Since it allows for less diffusion of lithium ions as compared with conventional electrodes used in lithium ion batteries, the high-rate properties and low-temperature properties of the battery become poorer as the porosity is reduced. A high energy density can be achieved, but the battery characteristics do not reach the practical level.